Hard disk drive memory systems have been used in the field of magnetic recording for storage of digital information for many years. In modern disk drive technology digital information is recorded on concentric memory tracks of a magnetic disk medium. The disks themselves are rotatably mounted on a spindle. A magnetic head or transducer is disposed on the surface of the disk for transferring information (i.e., reading/writing) to/from the disks as they rotate at a high speed. The rotation of the rigid magnetic disks causes the magnetic heads to be hydrodynamically lifted above the surface of the recording medium. This hydrodynamic lifting phenomena results from the flow of air produced by the rotating magnetic disk. It is this air flow which causes the head to "fly" above the disk surface.
The current trend in the industry is toward increasing the data storage capacity of magnetic recording systems while maintaining or decreasing the physical size of the units. This has been achieved, in part, by lowering the slider flying height. Generally, this means that the separation between the head and disk must be reduced. For instance, very low flying heights on the order of 1-3 microinches are becoming increasingly common. Proposals for future drive assemblies include "in-contact" magnetic recording systems, wherein the head is in continuous contact with the surface of the magnetic medium.
In the quest to increase the data storage capacity of hard disk drive assemblies practitioners are faced with a number design problems. Among these are the need to minimize the introduction of environmental contaminants into the drive unit and the need to maintain operating temperatures within acceptable limits.
To satisfy these environmental needs, a typical hard disk drive assembly includes a cover and a baseplate that form a housing when attached. A seal between the cover/baseplate interface prevents environmental debris from entering the enclosure. Obviously, the accumulation of environmental debris within the enclosure is highly undesirable since it causes an increased wear rate on the disk. Environmental debris can also lead to random signal modulation.
The need to eliminate environmental debris from the interior of the assembly housing becomes imperative in "near-contact" and "in-contact" magnetic recording systems. As might be expected, when flying heights diminish, the magnetic head becomes more susceptible to the influence of the surface condition of the magnetic disk. Any environmental debris present on the disk surface might cause catastrophic damage in such systems.
In addition to heightening the need for better contamination control, the use of high capacity disk drives also introduces problems relating to power consumption and temperature control. While lower capacity disk drives generally operate at 3600 rpm, high capacity drives--in order to improve transfer rates--typically rotate the disk pack at speeds between 6300 and 7200 rpm. Higher operating speeds require an increase in drive motor input power which, in effect, results in higher operating temperatures. In addition, higher disk speeds create more windage which, in turn, increases the air resistance forces within the drive unit. Because the assembly drive and actuator motors must ultimately overcome these tribological forces, they require additional input power. The net effect is that high capacity drives require more power and operate at higher temperatures than their low capacity counterparts.
As the temperature of a disk drive rises it also radiates a larger amount of heat. Since hard disk drives are often situated in computer systems in close proximity with electrical boards, it is desirable to minimize the amount of heat dissipated by the units. Excessive heat not only reduces the component life of the drive unit, but can also cause electric circuits, chips and other computer components to malfunction.
One approach that has proved successful in reducing the operating temperature of high capacity disk drives has been the use of hermetically sealed units filled with an inert gas. Lower operating temperatures are achieved by reducing the tribological forces acting upon the disk drive mechanisms. The method includes evacuating all air from the drive housing and injecting it with an inert gas, such as helium or nitrogen, at a pressure between 7 to 8 psi. The use of a hermetically sealed chamber does, however, make the repair of internal components virtually impossible since the chamber junction must be cut in order to access the components throwing dust and debris into the chamber.
Another approach for sealing the cover/baseplate interface has been to use a wide, open-cell, flat gasket. However, since an open-cell gasket is incapable of providing a pressure seal, it is inappropriate for use in pressure filled chambers. Conventional gaskets are also prone to leakage at various points around the periphery of the enclosure.
Yet another sealing method entails the use of a closed-cell, elastomeric o-ring disposed within a groove provided in the disk drive baseplate. Once the o-ring is positioned in the groove, the cover is attached to the baseplate, thereby compressing the o-ring such that a point contact is made about the periphery of the cover/baseplate interface. Although closed-cell o-rings are capable of providing a pressure seal, there are several problems associated with their use.
Generally, o-rings work well and are cost effective in applications where sealing diameters are greater than 0.05 inches. However, when sealing diameters fall below 0.05 inches, gasket compression ratio problems may arise if tolerances are not held tightly within specification. For example, if the tolerance range is too large the gasket may become under-compressed, resulting in a poor seal. Conversely, if the tolerance range is too small the gasket may become over-compressed, eventually causing the gasket to lose its sealant properties over time. Maintaining size tolerances within tight specification limits is very costly and is a problem in high volume manufacturing.
Another problem associated with the use of o-rings is that it provides only one point of sealing contact separating the internal components of the disk drive from the ambient environment. One contact point is generally insufficient to ensure a proper seal; particularly when the seal is used to contain a pressurized gas.
Another problem associated with the o-ring approach is that o-rings of irregular shapes (i.e., not having an "O" shape) tend to curl and become dislodged from the baseplate groove when the cover is attached to the baseplate during the manufacturing process. Consequently, this method often results in a poor seal.
What is needed then is a solution to the numerous problems inherent in the prior art sealing technologies. As will be seen, the present invention provides an apparatus and method for enclosing a hard disk drive while solving the aforementioned problems.